This invention relates to a disc brake adapted to be operated by a dual control liquid pressure system. In many vehicles, in order to obtain a safe and positive braking operation, brakes adapted to be operated by dual control liquid pressure systems have been and are being employed.
FIG. 1 of the accompanying drawings shows one of the prior art brakes of the above type in which a stepped piston is provided and in which during braking operation, liquid under pressure is supplied into hydraulic chambers A.sub.1 and A.sub.2 from a common master cylinder. When the brake is to be released, such release operation relies upon the elastic deformation or roll back of seals S.sub.1 and S.sub.2. Thus, in order to obtain rapid and positive separation between the disc and friction pads after a particular braking operation, the design preferably provides that all the seals between the piston and caliper produce a force due to the so-called roll back so as to retract the piston to its initial or non-application position, to thereby improve the operation efficiency. However, if only the seals S.sub.1 and S.sub.2 are employed (in which case, there would be no drain D provided because the liquid under pressure from the hydraulic chamber A.sub.2 would tend to flow out through the drain if any) and the seal S.sub.2 becomes damaged, although the liquid under pressure tends to flow out between the piston and caliper whereby damage on the seal S.sub.2 can be detected, when the seal S.sub.1 becomes damaged, communication is established between the hydraulic chambers A.sub.1 and A.sub.2, but damage to the seal S.sub.1 can not be detected. Therefore, in the prior art disc brake of FIG. 1, an additional intermediate seal S.sub.3 is positioned between the hydraulic chamber A.sub.2 and seal S.sub.1 and a drain D is provided extending from between the seals S.sub.1 and S.sub.3 to the exterior of the device. According to this design, if and when the seal S.sub.1 becomes damaged, the liquid under pressure within the hydraulic chamber A.sub.1 is prevented from flowing into the hydraulic chamber A.sub.2 by the presence of the additional intermediate seal S.sub.3 and is diverted to the exterior of the device through the drain D whereby damage to the seal S.sub.1 can be detected. However, the prior art disc brake as shown in FIG. 1 has the disadvantage that the roll back effect provided by the seals S.sub.1 and S.sub.2 is impeded by the seal S.sub.3. In other words, the seal S.sub.3 itself has no roll back effect (the roll back effect can not be obtained unless the direction of liquid pressure from the hydraulic chamber A.sub.2 to which the seal S.sub.3 is subjected corresponds to that in which the piston advances when the disc brake is applied) and thus, the resistance offered by the seal S.sub.3 tends to impede the roll back effect provided by the seals S.sub.1 and S.sub.2 and, thus, the sliding movement of the piston.
FIG. 2 of the accompanying drawings shows another prior art disc brake in which the piston employed is a cup-shaped piston and the additional intermediate seal S.sub.3 as shown in the prior art disc brake of FIG. 1 is eliminated because provision of the intermediate seal is difficult in the device of FIG. 2. When the disc brake is released, the piston is retracted to its initial or non-application position by the roll back effect. The seal S.sub.5 which separates the hydraulic chambers A.sub.3 and A.sub.4 is subjected to pressure on the opposite sides thereof or from the hydraulic chambers A.sub.3 and A.sub.4, and as a result, the seal S.sub.5 does not provide any roll back effect to thereby impede the roll back effect provided by the seal S.sub.4.
Therefore, in the above two prior art piston arrangement systems, it was difficult to obtain both proper designing of the additional or intermediate seal and proper provision of the roll back effect of the seals.